Since 2011 OWI-lab has been conducting acceleration measurements on an offshore wind turbine with a monopile substructure at the Belwind windfarm. One of the original research interest was the assessment of the resonance frequency of the substructure. In particular the aim was to compare the measured values with design estimates. Initial results confirmed that the actual resonance frequency was well above the as-designed values. An observation that was confirmed with additional measurements at Northwind and Belwind. Other researcher noted similar observations at different wind farms, concluding that in design the soil-stiffness was underestimated. At Belwind and Northwind a continued monitoring campaign now allows to track the evolution of the resonance frequencies over time. This set-up was intended for the detection of scour, which would lower the resonance frequencies. However, surprisingly, over the course of several years the resonance frequencies of Belwind have increased. This contribution considers these measurement results and compares them with a simplified model of the offshore wind turbine. To assess which changed boundary conditions of the offshore wind turbine would alter the resonance frequencies as observed at Belwind. In particular this contribution will focus on the possibility of soil-stiffening and soil-densification at the site, which are known mechanisms that alter the boundary conditions.

Method

Starting from measurements of accelerations on the wind turbine an operational modal analysis algorithm is used to continuously determine the resonance frequencies of the wind turbine and its substructure. In combination with the SCADA data and the sea-state the obtained resonance frequencies are corrected for the effects of the tidal level and the temperature. In these corrected results an increase of the resonance frequency over time was detected. By comparing the results from the monitoring with a simplified finite element model of the wind turbine it is possible to verify which change in the boundary conditions can explain the observed increase. In particular the effects of passing sand-dunes, soil-stiffening and soil-densification at similar sandy soils will be investigated.

Results

Measurements pointed out an increase of the higher order resonance frequencies over time. Hinting that the boundary conditions in the soil have changed. Lab-experiments already found soil-stiffening through soil-densification as a possible mechanism for sandy soils. However, these experiments were never put next to actual measurements offshore. In this contribution we will investigate the impact of these changed boundary conditions on higher order modes using a simplified model of the wind turbine and analyze if they match with the observations offshore.

Conclusions

For the first time the temporal evolution of resonance frequencies of offshore wind turbines was detected. The observations made will be discussed and assessed with a model. Using the model it will become possible to link the observed temporal behavior to actual changes in the boundary conditions. As the research is still ongoing the definitive conclusions are still investigated. However incorporating the temporal behavior of the soil into the design can help to improve design. In particular towards fatigue, for which an increased resonance frequency can be beneficial. And towards rotor-structure interactions that can occur when the resonance frequencies shift into the 3p and 6p frequency bands of the rotor. Such an interaction can lead to elevated vibration levels

Objectives

From this presentation the delegates will get a better understanding on the possibilities to use the (higher order!) resonance frequencies of offshore wind turbines as a monitoring tool for the soil conditions, a.o. scour. Results will illustrate the temporal behavior of the soil and of the boundary conditions in general which can serve as input for design engineers.